From the state of the art, different solutions for start-stop designs for internal combustion engines are known that are used, in particular, for saving fuel. In the case of a short-term stop, for example, a vehicle stopped in front of a red light or in a traffic jam, these designs stop the supply of fuel, whereby the operation of the combustion engine is interrupted. With such designs, the fuel consumption can be reduced by, for example, 3% to 5% relative to the new European driving cycle (NEFZ). Simultaneously, however, the realization of a start-stop design considerably increases the startup frequency to be specified. Under typical driving conditions, the expected startup frequency of a combustion engine of a motor vehicle increases from 50,000 to 3,000,000. In addition, start-stop designs initially lead to increased noise emissions, for example, an increase in the NVH level (Noise Vibration Harshness) in a motor vehicle. If the start-stop design is realized with the help of an electric starter motor, then the starter motor must be designed for the increased startup frequency. The starter motor must have larger dimensions, whereby the costs and mass of the starter motor increase. In addition, the power supply network in the motor vehicle is loaded for each startup process, whereby the alternator and the battery of the motor vehicle also must have larger dimensions. In addition, for each startup process, electrical energy must be expended, whereby the potential energy savings of the start-stop design are reduced.
Furthermore, from the state of the art, solutions are known in which the electrical starter motor is replaced partially or completely. Here, the startup of the combustion engine is realized by the ignition of fuel located in at least one of the cylinders of the stopped combustion engine. Such a startup is also designated as a direct startup. To be able to perform a direct startup, the piston of each cylinder must be located within a narrow crankshaft window that is set approximately 100° after the top dead center of the crankshaft.
From DE 103 51 891 A1, a method and a control device for restarting an internal combustion engine are known. The restart is realized during a current operating time period after the internal combustion engine had already been turned off at the end of a preceding operating time period. To allow the restarting of the internal combustion engine without a starter motor, fuel is initially injected into the intake pipe of the internal combustion engine, when its cylinder is located in an intake cycle during the last full revolution of the preceding operating time period, so that the combustion space of the cylinder is then filled with a fuel-air mixture. Furthermore, the piston of the cylinder transitions at the end of the preceding operating state into a standstill position in which the piston assumes a work cycle position and the fuel-air mixture remains enclosed in the combustion space. Finally, the restarting of the internal combustion engine is realized through the ignition of the fuel-air mixture enclosed in the combustion space, as long as the mixture is still combustible. One disadvantage of this method is that the transition of the piston of the cylinder into a standstill position requires precise positioning of the piston, wherein work for compressing the fuel-air mixture is to be performed.
EP 1 586 765 A1 shows a method and a control system for positioning a crankshaft when a combustion engine is turned off. In this way, an electric motor coupled with a crankshaft of the combustion engine is controlled, in order to position the crankshaft at a startup angle. The control system comprises a detection device for receiving and/or determining a positional angle and/or the rotational speed of the crankshaft and also a control unit that is suitable to control an electric motor in a control operating mode as a function of the rotational speed of the crankshaft and to move the crankshaft to the startup angle after the combustion engine stops. The combustion engine is started from this startup angle, wherein the control unit is configured so that the electric motor is controlled according to a control operating mode below a given limiting rotational speed in a controlled operation independent of the rotational speed, so that the crankshaft is adjusted to the startup angle. This solution has the disadvantage that the positioning of the crankshaft when the combustion engine is turned off is complicated and additional energy is also required for this positioning.
DE 197 43 492 A1 shows a method for starting an internal combustion engine, in particular, of a motor vehicle. This method is provided for an internal combustion engine that is provided with a piston that can move in a cylinder and that can pass through an intake phase, a compression phase, a work phase, and an exhaust phase. The fuel is injected directly into a combustion space defined by the cylinder and piston either in a first operating mode during the compression phase or in a second operating mode during the intake phase. According to this previously known method, for an initial injection the fuel is injected directly into the combustion space whose associated piston is located in the work phase. The power that can be generated with this method, however, is inadequate for starting the internal combustion engine in many cases.
From DE 10 2006 012 384 A1, a startup method for an internal combustion engine with direct injection is known that has, in a hybrid vehicle, an externally ignited combustion engine and an electric motor for its drive mechanism. The combustion engine is started from standstill without mechanical excitation, in that, with the electric motor, for driving the hybrid vehicle, at least one cylinder is brought into a suitable startup position between 30° and 150°, advantageously with a crank angle of between 70° and 90° according to its top dead center after the compression phase. A quantity of fuel is injected into this one or more cylinders and ignited after the injection. This method for realizing the start-stop design is limited to hybrid vehicles.
From DE 10 2004 046 182 A1, a method for the combustion-assisted start-stop operation of an engine with cylinder-valve shutdown is known. This method initially comprises a step for setting a throttle valve, in order to provide a mass flow of air to an engine of a vehicle that is sufficient for creating startup torque. Furthermore, the fuel that is sufficient for startup torque is injected into the cylinder of the engine during an intake cycle of a cylinder. In addition, the operation of a spark plug is interrupted and an exhaust valve and an intake valve of the cylinder are blocked. In this state, the engine is turned off. For turning on the engine, a piston of the cylinder is positioned between a top dead center of a compression cycle and a bottom dead center of a work cycle or between a top dead center of a discharge cycle and a bottom dead center of an intake cycle. A fuel-air charge that is sufficient for creating startup torque is ignited in the cylinder with the help of the spark plug. Finally, the intake valve and the exhaust valve are activated. This method also requires complicated positioning of the piston during the shutdown process of the engine, wherein mechanical work is required for compressing the fuel-air mixture. In addition, during the startup process it cannot be guaranteed with this method that the combustion of the fuel-air mixture is performed to a large degree ideally, so that the pollutant emissions are increased during the startup process.